The dendritic cell is a recently recognized new cell type having important immunological functions. It provides accessory activity required for responses of T lymphocytes, acts as a potent stimulator of a mixed leukocyte reaction and, owing to its richness in Ia antigens, serves as a critical "passenger cell" that elicits rejection of transplanted tissues. Dendritic cells arise from precursors in bone marrow by a process that is the major focus of this research proposal. Virtually all precursors are recovered in a low density fraction (LD-BMC) that provides a 20-fold enrichment of these cells. Studies in vitro with LD-BMC indicate that, unlike the dendritic cell, the precursor is a dividing cell and is deficient or devoid of Ia. In cultures of LD-BMC, precursors develop over a five day period into dendritic cells that are functionally and morphologically indistinguishable from dendritic cells isolated from tissues. A soluble factor(s) produced by mitogen-treated lymphoid cells markedly augments the production of dendritic cells. Proposed studies aim at defining the role played by various cell types in the development of dendritic cells from precursors in a liquid culture system. Specific immunoadsorption and rosetting procedures will be applied. Autoradiography, immunofluorescence, and functional tests will be used to determine the kinetics of formation of Ia+ dendritic cells capable of accessory and stimulatory activity. The factor(s) responsible for augmenting dendritic cell production will be purified for the purposes of characterization and determining whether it acts directly on the precursor or on some other regulatory cell. The factor will also be used in an agar system for studies on the frequency, composition, and renewal of colonies of dendritic cells. An attempt will also be made to propagate dendritic cells. With bone marrow replacement increasingly used as a therapeutic tool, a greater knowledge of dendritic cell dynamics may contribute to making it a more efficacious procedure. No studies have been performed on the development in vitro of dendritic cells, and our research will provide new information about this process.